The present invention relates to a storage system employing a rewritable nonvolatile memory device.
In organizations such as enterprises, in order to manage large quantities of data, a storage apparatus is employed that is separately constructed from the host computer (hereinbelow referred to as “host”). For example, in organizations such as financial organizations or hospitals, it is necessary to store for example trading data relating to a large number of customers or diagnostic data over a long period, so storage devices of high reliability and large capacity are required. As such storage apparatus satisfying both requirements for high reliability and high capacity, for example a host may be provided with a logical storage region using a RAID (redundant array of independent disks) in which a large number of hard disk drives are connected in array fashion.
The amount of data to be managed long-term by enterprises etc increases daily. Consequently, the number of hard disk drives mounted in a storage apparatus continues to increase. In a hard disk, as is well known, reading of data is achieved by causing a magnetic head to perform seeking while rotating the magnetic disk at high speed using a spindle motor. The external dimensions and weight and also the power consumption of a hard disk drive are therefore larger than in the case of a storage apparatus such as semiconductor memory.
As the storage capacity of a storage apparatus is made larger, the number of hard disk drives mounted therein increases. Consequently, the installation area of the storage apparatus increases, and the power consumption of the storage apparatus becomes large. The cost of introduction and the cost of maintenance of the storage apparatus therefore increase, tending to raise the total cost of ownership (TCO).
In recent years, a new type of a storage device called a flash memory device has attracted attention. Comparing a flash memory device and a hard disk drive, the flash memory device has lower power consumption and a faster speed of reading data. Also, since flash memory devices do not need mechanical moving parts, they can be formed of smaller size than hard disk drives, and have the property that failure is unlikely to occur.
Flash memory devices are rewritable nonvolatile memories capable of continuing to hold data for a comparatively long period. However, in flash memory devices also, due to the gradual change in the voltages held in the memory cells due to leakage current, if the flash memory device is left undisturbed for a long period, there is a possibility that it will before long become impossible to read data from the memory cells.
Accordingly, the technique has been proposed of refreshing data stored in a flash memory device by writing the data back to its original location after first reading the data stored in a flash memory device (specification of Laid-open US Patent Application No. 2005/0243626).
The prior art set out in the above reference aims to achieve long-term storage of data by refreshing of data in a flash memory device. However, the prior art set out in the above reference aims solely at refreshing unit flash memory devices and no consideration is given to a storage apparatus comprising a large number of storage devices. The prior art set out in the above reference cannot therefore be directly applied to a storage apparatus.
As described above, it is necessary that a storage apparatus should continue to hold important data for a period ranging from some years to some tens of years. Consequently, in contrast to home-use products such as for example music players using flash memory devices, an extremely high level of reliability over a long period is required. Also, as described above, a storage apparatus requires a large number of storage devices for managing large quantities of data. A mechanism for using a large number of flash memory devices in an efficient and stable fashion is therefore required.
The memory capacity required by a storage system increases as the amount of data to be managed increases. In order to increase the storage capacity of a storage system, a storage system may therefore comprise a plurality of storage apparatuses. A storage system may therefore include an assortment of storage apparatus of respectively different specifications and it is necessary to preserve important data for a long period in a heterogeneous environment.
The case may therefore be envisioned in which although a certain storage apparatus has a data refresh function, other storage apparatus does not have a refresh function. Also, even in the case of memory devices mounted in the same storage apparatus, the case may be envisioned in which some memory devices incorporate a refresh function whereas other memory devices do not have a refresh function.
In storage systems comprising a plurality of storage apparatuses respectively constructed by large and complex system products, increased storage capacity combined with high reliability permitting no loss of data are demanded, and also ceaseless operation (24 hours a day and 365 days a year) is sought. Since storage systems comprising a storage apparatus have these technical features, the performance and reliability required by the market cannot be satisfied simply by applying the prior art set out in the reference given above.